1. Field of the Invention
The present invention relates to a display device including a thin film transistor. In particular, the present invention relates to a liquid crystal display device or a light-emitting device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices and have been rapidly developed particularly as switching elements for image display devices such as a liquid crystal display device.
In a semiconductor element typified by a thin film transistor, it is one of important objects in manufacturing steps of a semiconductor device that how an electrification phenomenon (charging) which leads to deterioration in an element or dielectric breakdown is suppressed. In particular, since film thickness of various insulating films such as gate insulating films is decreased in accordance with high integration, dielectric breakdown due to charging has become a more serious problem.
Causes and circumstances of charging are extremely complicated and diverse. Therefore, it is necessary that causes and circumstances of charging be found out. In addition, it is necessary that the structure of a semiconductor device itself be devised so that resistance against deterioration or dielectric breakdown due to charging is increased. In order to prevent deterioration or dielectric breakdown due to charging, it is effective to provide a discharging path by a protection circuit formed using a diode (a protection diode). When a discharging path is provided charge accumulated in an insulating film can be prevented from being discharged near a semiconductor element, so that a phenomenon where the semiconductor element deteriorates or is damaged by discharge energy (ESD: electrostatic discharge) can be prevented.
Further, when the protection circuit is provided, even when noise as well as a signal and power supply voltage is input, a malfunction of a circuit due to the noise can be prevented and deterioration or damage of the semiconductor element due to the noise can be prevented.
In an image display device such as a liquid crystal display device, a thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film is mainly used as a switching element.
As a method for forming a polycrystalline semiconductor film, a technique is know in which a pulsed excimer laser beam is processed linearly by an optical system and an amorphous semiconductor film is irradiated with a linear laser beam while being scanned by the linear laser beam to be crystallized.
Further, as well as a thin film transistor using an amorphous semiconductor film or a thin film transistor using a polycrystalline semiconductor film, a thin film transistor using a microcrystalline semiconductor film is known as a switching element of an image display device (for example, see Reference 1: Japanese Published Patent Application No. H04-242724, Reference 2: Japanese Published Patent Application No. 2005-49832, Reference 3: U.S. Pat. No. 4,409,134, and Reference 4: U.S. Pat. No. 5,591,987).
As a method for manufacturing a thin film transistor using a microcrystalline semiconductor film, a technique is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with diode laser to modify the amorphous silicon film into a microcrystalline silicon film. With this manufacturing method, the metal film formed over the amorphous silicon film only converts light energy of the diode laser into thermal energy and is removed in a later step. That is, the amorphous silicon film is heated only by conduction heating and the microcrystalline silicon film is formed by this heat (for example, see Reference 5: Toshiaki ARAI and others, SID '07 DIGEST, 2007, pp. 1370 to 1373).